electrophilicreactions of aromatic compounds,and the unusually large one must conclude that the electroreactivity of the ortho-positions, philic electron looalisationenergies decreaee strongly in the immediate neighbourhoodof the fluorine-substituted carbon atom. Therefore,if one assumes 1,2 that the determiningstep in the carcinogenicprocess in an aromatic hydrocarbonis an electrophilic reaction of the "K" region, and if one compares the electrophilicelectron localisationenergies of the compounds (a) with an unsubstituted"K" region and (b) with fluorine in the "K" region, one arrives at the following predictions: First possibility:if the electrophilicsubstitutiontakes place only at the fluorinatedcarbon atom, the carcinogenicactivity will